An apparatus having a plural number of transducer composed of a two-dimensional transducer array in which sector scanning is performed using an ultrasonic beam (in an arbitrary direction) generated by a transmitting/receiving circular pattern of transducers formed in the two-dimensional transducer array is known. In this ultrasonic diagnostic apparatus, for example, if 64×64 transducers are disposed in the two-dimensional array, then the total number of transducers is 4,0967; and, ultrasonic scanning is performed with a separate delay control provided for each transducer element. In this case, a beam-forming circuit with 4,096 channels is necessary. Realizing a beam-forming circuit having such a large number of multi-channel delay circuits is difficult. So, by thinning out the number of driven elements for forming one ultrasonic beam, the number of delay circuit channels in the beam-forming circuit is reduced. But the S/N of signal acquired by thinning out the driven elements is deteriorated. So, the apparatus is realized by comprising a beam-forming circuit having as many as delay channels possible. For example, there is an apparatus that comprises a beam-forming circuit having a delay circuit of 256 channels for transmitting and 256 channels for receiving.
As shown in FIG. 4, a linear scanning expanded field of view obtained by moving the transmitting/receiving circular pattern of transducer elements 1, that are arrayed in a two-dimensional X, Y direction, and a scanning method in which a convex scanning is applied to two-dimensions are proposed. In this case, the number of transducer elements 1 is more than in said sector scanning type apparatus. So, for reducing the number of channels in the beam-forming circuit, an apparatus is known that produces an ultrasound transmitting/receiving circular pattern 2 by electronically bundling a plural number of transducer elements 1 in the two-dimensional array into a multi-ring arrangement with concentric rings, so as to give a consistent delay time to the transducer elements composing one ring of said multi-ring arrangement. The apparatus transmits/receives an ultrasonic beam with a delay time between rings, and forms an ultrasonic image by moving said circular pattern 2 in X, Y directions.
As shown in FIG. 5, in said multi-ring arrangement, each ring is formed by the bundling of transducer elements in concentric rings of which the distance L1, L2 . . . , from the single focal spot F is almost the same, and the diameter of the most exterior ring is diameter 2 for ultrasound transmitting/receiving. In this method, the bundling of transducer elements in concentric rings makes it possible to reduce greatly the number of channels in the beam forming circuit, which corresponds to the number of delay circuits, and the S/N of a signal acquired by using all elements in a circular pattern can be improved.
When considering the shape of an ultrasonic beam, a focusing calculation for an ultrasonic beam in the object to be examined is traditionally performed under the condition that the speed of sound in an ultrasonic propagation medium is uniform. As shown in FIG. 6, a traditional apparatus comprises delay circuits 4, 4, . . . comprising one per transducer element of the probe 3 having a plural number of transducers and an adder 5 for adding received signals output from these delay circuits 4, 4, . . . . Although reflected signals from focus point 6 in the object propagate through medium 7 to reach each transducer element, the difference in the path length from the focus point to each transducer element causes a difference in the arrival time of each reflected signal. In this case, the reflected signals reach the transducer elements located in the center of probe 3 early, and, on the other hand, they reach the transducer elements at the ends late. So, the shape of the wave surface 8 in the received signals is not linear. Thus, the signals output from transducer elements in the center part of the probe are delayed with a large amount of delay time in delay circuits 4 corresponding to each received signal, and the signals output from transducer elements at the ends of the probe are delayed with a small amount of delay time. The signals are then output to adder 5. With these delay operations, the wave surface 9 of the signal output from delay circuits 4 is linear since the signals have the same phase. The received signals having the same phase, such as shown by wave surface 9 in this condition, are added in adder 5 so as to form the combined signal 10.
But actually, as shown in FIG. 7, a sound speed non-uniformity part 11 typically exists on the path from the focus point 6 in the object to the probe 3, so that the wave surface of the received signal is disturbed, as shown at 8′. In this case, when performing the delay operation, while assuming that the speed of sound is uniform, the wave surface of the received signals output from delay circuits 4 is distorted. as shown at 9′, so that they do not have the same phase. Accordingly. the output produced in adder 5 does not increase in intensity as the signals are added, so that its intensity is small, as shown by signal 10′.
On the contrary, there is a technology referred to as an adaptive ultrasonic imaging method which operates to correct the delay amount produced in said delay circuits 4 in accordance with the speed of sound in the medium. In the adaptive ultrasonic imaging technology, a mutual correlation method for correcting the delay amount by correction processing of respectively received signals between adjacent channels, and a maximum value brightness method for searching for a brightness maximum while changing the delay amount of the delay circuits are known.
FIG. 8 is a block diagram which illustrates the mutual correlation method. In FIG. 8, a signal received from each transducer element, which is not shown in the figure, is delayed by a predetermined amount by a respective delay circuit 4, 4, . . . . This delay is possible by use of analog delay circuits or digital delay circuits. In this case, when outputs of adjacent channels in each transducer element, a mutual correlation processing is carried out with correlation device 12, and the phase difference between the outputs of adjacent channels can be obtained. By detecting the phase difference value, transforming it to focus data in correction processing part 13, and feeding the transformed data back to focus controlling part 14, the delay amount produced by the individual delay circuits 4 can be corrected.
FIG. 9 is a block diagram illustrating the maximum value brightness method. In FIG. 9, a received signal from each transducer element, which is not shown in the figure, is delayed in a respective delay circuit 4, 4, . . . by a predetermined amount. The outputs delayed in the respective delay circuits 4, 4, . . . are added in adder 5., and this output is input to maximum value detecting part 15. This maximum value detecting part 15 compares the input signal with the last input value, and, in case the input value is smaller than the last detected value, the focus data is slightly changed systematically in focus controlling part 14. Then, after the phasing of the received echo signal has been changed by this focus data, the output of adder 5 is inputted to the maximum value detecting part 15, and judged again. After repeating this operation, when the detected value is formed to converge at the maximum value, its data is used as focus data.
However, as shown in FIG. 4 and FIG. 5, in an ultrasonic diagnostic apparatus in which a transmitting/receiving circular pattern 2 is formed by bundling transducer elements of a two-dimensional array into concentric rings to compose a multi-ring arrangement, in case there is a sound speed non-uniformity part 11 on the path from focus point 6 to the probe 3 in the object, since transducer elements 1 in the two-dimensional array are bundled in concentric rings as thus described, it is difficult to detect the phase difference due to said path difference for correcting for the influence of said sound speed non-uniformity part 11. Therefore, the phase difference of echo signals caused by said path difference, due to the existence of the sound speed non-uniformity part 11, is not corrected. As a result, the image quality is deteriorated because the ultrasonic beam becomes worse.
Thus, it is an object of the present invention to solve the above-mentioned problems by providing an ultrasonic diagnostic apparatus which is able to correct a focusing error by detecting the phase difference of echo signals which occur due to a difference in the ultrasonic propagating path, even when its multi-ring arrangement of transducers is composed by bundling transducers in a two-dimensional array for transducer elements in concentric rings.